Planar distributed periodic structures more commonly known as mushroom struc- tures which present a negative refractive index (NRI) in a very narrow frequency band, are usually studied using equivalent electric models under quasi-static approximation. The dispersion equa- tions for di®erent con¯gurations can be established using Bloch theorem for 2D transmission lines (TL). This paper proposes a critical study concerning the validation of results obtained from these utilised dispersion equations through comparing them to those obtained using a full- wave method. An extraction method has been proposed to simulate the frequency dependent of each distributed element. these results shown a frequency dependence di®erent from that ob- tained from e®ective elements calculated using conventional analytical formulas for the e®ective material parameters of the unit cell.